Posttranslational histone modifications are critical for the regulation of genome function. The levels of histone modifications oscillate during the cell cycle. Most modifications are diluted after DNA replication and then their levels are restored during the rest of the cell cycle with different kinetics depending on the modification. Some modifications, like histone H4 Lys20 monomethylation (H4K20me1), exhibit cell cycle-dependent dynamic changes. To track histone modifications in living cells, we have developed genetically encoded probes termed modification specific intracellular antibodies, or "mintbodies." As mintbodies shuttle between the cytoplasm and nucleus by diffusion, their nuclear concentration depends on the target modification level. By measuring the nuclear to cytoplasmic intensity ratio of H4K20me1-specific mintbody, we have monitored the increase of H4K20me1 in the G2 phase. Here we describe how the mintbody-based methods can be applied to track a specific chromosome, such as the inactive X chromosome (Xi), on which genes are repressed through histone H3 Lys27 trimethylation (H3K27me3). When H3K27me3-specific mintbodies are expressed in cells that harbor Xi, the mintbodies are concentrated on Xi and the dynamic behavior of Xi can be tracked using a confocal microscope. After acquiring 3D time-lapse images, an image analysis allows measuring the volume, shape and H3K27me3 level of Xi during the cell cycle.
Keywords: Chromosome territory; Histone modification; Intracellular antibody; Live-cell imaging; X chromosome inactivation.